Comparison of behaviour of rare earth containing catalysts in the oxidative dehydrogenation of ethane

Catalyst promotion by addition of either La and Sm to MgO or Na aluminate to Sm2O3 and La2O3 has been investigated for the oxidative dehydrogenation of ethane in the temperature range 550-700 degrees C. With all unpromoted and promoted catalysts, the selectivity to ethylene is strongly enhanced by the temperature, the highest values being obtained at 700 degrees C. Sm2O3 is the most active among the bulk oxides. while samarium addition to MgO results in higher surface area, but does not enhance the catalytic activity. Ethylene productivity on La2O3 promoted MgO samples is higher than with pure La2O3, Sm2O3 and MgO, not only due ro the stabilising effect of La on MgO surface area, but also due to a higher intrinsic activity. With both bulk oxides and rare earth promoted MgO, the selectivity to ethylene strongly increases by decreasing the O-2/C2H6 feed ratio, while it is quite unaffected by ethane conversion and catalyst composition. in agreement with the hypothesis that the main role of catalyst in the experimental conditions investigated is to produce ethyl radicals which are converted in the gas phase to CO and C2H4. When La2O3 is modified by the addition of sodium aluminate the catalytic behaviour significantly changes, Likely due to a different, mostly heterogeneous reaction mechanism. On aluminate promoted lanthana, ethane is converted to ethylene with higher yields which do not depend on the feed ratio. Moreover. only CO2 is produced as by-product, the formation of CO being quite negligible. (C) 2000 Elsevier Science B.V. All rights reserved.